This invention relates to a circuit to compensate for switching speed variations of semiconductors and more particularly to such a circuit which automatically adjusts the turn-on time of transistors which apply fixed current pulses to a load.
An often used technique for controlling the torque motors of inertial instruments, for example, is pulse width modulation in which rapidly alternating current pulses are applied to the motor. When the durations of the positive and negative pulses are equal, the net torque delivered to the motor is zero; non-zero torques are produced by altering the ratio of positive to negative pulse duration. For accurate operation of precision inertial instruments, it is critical that when zero net torque is commanded, the actual net current delivered to the motor, if not identically zero, be very stable. Any such non zero current is known as bias current and variation in it degrades instrument performance.
The positive and negative pulses are typically created by switching semiconductors such as transistors from a conducting to a nonconducting state and vice versa. Often, the transistors are arranged in an H-bridge configuration to effect this switching of the pulses from positive to negative. Because semiconductors require a finite time to change from the conducting to the nonconducting state, a small current transient appears with each current reversal. The shape of the transient waveform is related to the bias current, and, therefore, changes in the transient waveform shift the bias current.
The major factor which alters the transient waveform and hence also the bias current is the instability in the time needed to turn off the transistor, that is, the time it takes for the transistor to change from the conducting to the nonconducting state. In this regard, it is known that the turn-on time, much faster than the turn-off time, is also more stable than the turn-off time. It is this instability in transistor turn-off time which most degrades performance of inertial instruments controlled by pulse width modulation.
It is an object of the invention disclosed herein, therefore, to provide a circuit to compensate for the switching speed variations of semiconductor devices thereby allowing the bias current level to be stabilized very accurrately.
A further object is to provide such a circuit which has few elements and which is inexpensive and straight forward to implement.